feature: Adds a screen to record an impromptu alarm ringtone.

[chlohal]

This is helpful to wake someone up with a different alarm every time, or to use speech with something specific to their scenario rather than a generic tone (which has been shown to be better at waking someone up and avoiding sleep inertia)

Not very sure about my interface design for this, so I would welcome any review or constructive criticism on that! This is also my first PR in Dart (along with #450 ), so I'm sorry if it's rough around the edges. I'm open to do any edits that you'd like in order to get it to project standards!